Finishing Basement - Insulating Walls and Floors

I am new to the forum and just scheduled my home inspection on my first house!

I have worked at Lowes for 10 years and that gives me about zero practical knowledge but 100% confidence that I can do a lot of stuff myself with the right advice and access to a lot of information.

The house I am looking at has 945 square feet of unfinished basement space. Built in 1988' it has poured concrete walls, 7' 3" of ceiling height (about 5 feet below ground), and when I move in I plan on starting the finishing project right away.

The basement has a French drain all around the perimeter and looks to be bone dry

I've been reading posts on here and other forums and I'd love to get some help/advice. Most of what I've used to formulate my plan has come from posts on this and other forums by "Dana" who seems to have a lot more technical knowledge than I can access anywhere else.

My plan was to stud directly to the concrete walls with PT 2X4 every 24" OC around the perimeter walls and then use 2" XPS adhered in between all around. I would take care to insulate near the sill plate. Then, because of ducting and headroom requirements, I was going to put 6 mil down throughout overlapped 12", 1" XPS on top of that, 1/2" plywood screwed down, and then carpet or laminate floor. Once I get my electrical in, I plan to use green board throughout on my studs.

A few questions:

1) are there any off gassing concerns using so much XPS?

2) how should I run the 6Mil to not create a problem for my French drain? I've read you should run it up the walls, but I don't know if that will cause an issue for the drain

3) does this sound like sufficient insulation for the walls/floor?

4) any mold concerns with this approach?

5) between the 2x4 and XPS, should I use spray foam to seal my seams or will Tyvek tape suffice. I have a concern about off gassing with the Great Stuff Foam in large quantities.

6)I was planning to keep a part of the basement as utility room and finish the walls but not the floors. Will this cause any moisture issues? I am planning on setting up 70 pint dehu's with humidistat set low.

Thanks in advance for any help. This is exciting and nerve racking all at the same time. The more I read the more I realize how many people make horrible mistakes with their renovation projects in their homes and mostly it's because they don't want to research or ask questions when they don't understand. Maybe THIS is what I've gotten out of working at Lowes for so long. LOL!

When I did this I adhered the XPS directly to the walls and framed regular stud walls inside of the XPS. I avoid any wood touching concrete, to provide a thermal break and reduce the chances of any mold/mildew/rot in the future.

Remember that you will need to run electric in these new walls, making full stud walls ideal.

I would use a pressure treated bottom plate, with tar paper under it - to avoid direct contact with the concrete floor - secure to the floor with lag bolts & anchors or blue screws.

I'd use regular 2x4 studs (not PT) - PT is very wet and can warp and move significantly when dried out. Install the top plate to the floor joists, and carefully measure each stud and attach the studs to the top/bottom plate. Also I'd do studs every 16" (in case you want to hang stuff up later). Plan your framing so there is some place to attach the dry wall (especially corners). I put double studs at the sheetrock seams - was easier to attach the drywall. Bought a nice used framing nailer at a Pawn shop for $75 - made the job a lot easier.

Use the pink XPS insulation. You'll notice the 4x8 sheets are scored to snap at 16". I snapped them, installed the corner stud, used the insulation as a guide, and marked the location of the next stud. Erected the stud, used the XPS as a stop - so I could toe nail the one side of the stud. You don't need to glue the XPS to the wall (assuming it's tight in the cavity.) Don't use any poly film (it will trap moisture) the XPS is your vapor barrier. You don't need any Great Stuff or tape - unless you have large, irregular gap. Don't make the wall super tight.

I reccomend the 4" square electrical boxes - you then buy a 1/2" mud ring (either single or double device). These boxes + mud ring give you considerable space for the wiring.

I'd use regular 2x4 studs (not PT) - PT is very wet and can warp and move significantly when dried out. Install the top plate to the floor joists, and carefully measure each stud and attach the studs to the top/bottom plate. Also I'd do studs every 16" (in case you want to hang stuff up later). Plan your framing so there is some place to attach the dry wall (especially corners). I put double studs at the sheetrock seams - was easier to attach the drywall. Bought a nice used framing nailer at a Pawn shop for $75 - made the job a lot easier.

Use the pink XPS insulation. You'll notice the 4x8 sheets are scored to snap at 16". I snapped them, installed the corner stud, used the insulation as a guide, and marked the location of the next stud. Erected the stud, used the XPS as a stop - so I could toe nail the one side of the stud. You don't need to glue the XPS to the wall (assuming it's tight in the cavity.) Don't use any poly film (it will trap moisture) the XPS is your vapor barrier. You don't need any Great Stuff or tape - unless you have large, irregular gap. Don't make the wall super tight.

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I thought you had to use PT for sill applications (I think my town requires it actually). Also, I understand that the PT dries out and can warp, but it will be fastened by the time this happens. The 16" on center is a good point and I thought the T-250 Xps was scored at 24"? Maybe I missed something. If its scored at 16" doesn't that leave 8" of waste? I literally was thinking of running with no sill plate around the walls and just dropping vertical studs every 24". My Sheetrock will still be securely fastened and will stop like and inch above the xps and plywood I'm doing on the floor. This ill hopefully prevent ANY moisture wicking from ANY type of sill plate.

Finally, I'm using 2" xps on the walls but was planning on using 2x4 to frame. Which measures nominally to 1.5". Hmm. Back to the drawing board I guess.

When I did this I adhered the XPS directly to the walls and framed regular stud walls inside of the XPS. I avoid any wood touching concrete, to provide a thermal break and reduce the chances of any mold/mildew/rot in the future.

Remember that you will need to run electric in these new walls, making full stud walls ideal.

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I really like the idea of studding ON TOP OF the xps, but I'm using 2" for the R-10 and can't really figure hom I'm going to attach them to the wall through 2" of foam. This would solve my depth issue with the 2" thick foam but 1.5" studs though.

You're way up north like me, (further even) what thickness foam did you use?

Assuming your basement is dry - I think you are getting a little carried away.

"I thought you had to use PT for sill applications (I think my town requires it actually). " - I said use PT for the bottom plate (that is the sill plate). You can use regular fir 2x4's for the wall studs. You need a bottom plate. The fir 2x4's are just fine against the block wall.

I believe the building codes (ask your inspector) require R-10. I believe that 3" of XPS is R-10. The studs should be attached to the top/bottom plate - you don't need to attach the studs to the wall. Forget that 'thermal break' stuff - it's wood. A stud has an R value of about 4.5 - which isn't bad - almost 1/2 of R10.

I am new to the forum and just scheduled my home inspection on my first house!

I have worked at Lowes for 10 years and that gives me about zero practical knowledge but 100% confidence that I can do a lot of stuff myself with the right advice and access to a lot of information.

The house I am looking at has 945 square feet of unfinished basement space. Built in 1988' it has poured concrete walls, 7' 3" of ceiling height (about 5 feet below ground), and when I move in I plan on starting the finishing project right away.

The basement has a French drain all around the perimeter and looks to be bone dry

I've been reading posts on here and other forums and I'd love to get some help/advice. Most of what I've used to formulate my plan has come from posts on this and other forums by "Dana" who seems to have a lot more technical knowledge than I can access anywhere else.

My plan was to stud directly to the concrete walls with PT 2X4 every 24" OC around the perimeter walls and then use 2" XPS adhered in between all around. I would take care to insulate near the sill plate. Then, because of ducting and headroom requirements, I was going to put 6 mil down throughout overlapped 12", 1" XPS on top of that, 1/2" plywood screwed down, and then carpet or laminate floor. Once I get my electrical in, I plan to use green board throughout on my studs.

A few questions:

1) are there any off gassing concerns using so much XPS?

2) how should I run the 6Mil to not create a problem for my French drain? I've read you should run it up the walls, but I don't know if that will cause an issue for the drain

3) does this sound like sufficient insulation for the walls/floor?

4) any mold concerns with this approach?

5) between the 2x4 and XPS, should I use spray foam to seal my seams or will Tyvek tape suffice. I have a concern about off gassing with the Great Stuff Foam in large quantities.

6)I was planning to keep a part of the basement as utility room and finish the walls but not the floors. Will this cause any moisture issues? I am planning on setting up 70 pint dehu's with humidistat set low.

Thanks in advance for any help. This is exciting and nerve racking all at the same time. The more I read the more I realize how many people make horrible mistakes with their renovation projects in their homes and mostly it's because they don't want to research or ask questions when they don't understand. Maybe THIS is what I've gotten out of working at Lowes for so long. LOL!

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First if you put the studs against the concrete you'll have a HUGE thermal bridge undercutting the performance of the foam, and provide a wicking path for moisture.

Instead, but the foam directly against the wall, foam-seal or tape the seams with housewrap tape, and trap the foam in place with a studwall. You can then insulate the stud bays with fiberglass or rock wool, as long as it is unfaced goods. (Don't use cellulose in this app- it's too hygroscopic for the app, in most basements.) If you're concerned about using up too much space, turn the studs sideways or use 2x2s- it's not a structural wall, it only needs to trap the foam to the wall and provide a means of hanging the gypsum.

If the finish floor is carpeting, you need to use at least 23/32" t & g subflooring, not half-inch plywood. You can get away with thinner stuff if the finish flooring itself has some structural strength, but with carpeting you're almost guaranteed to end up with some "leaf-twist", and warpage & lift at the seams if you cheap out with half-inch ply as the subfloor. A t & g product that's a bit stiffer would take care of that.

Also, be sure to foam-insulate and seal the foundation sill & band joist to the foundation-wall foam. The single largest infiltration hole in most existing homes is at the foundation sill & band joist, which is typically several times all window & door seam-leakage combined. Cut'n'cobbled foam in this area is best sealed with a closed cell foam such as Great Stuff or if you have a lot of it, FrothPak (the 12 board foot kits are pretty cheap compared to the amount of Great Stuff you'd be going through, and the blowing agent used isn't quite as hazardous to the installer.)

In answer to the questions:

1)There is some outgassing of both styrene and the blowing agent with XPS and some people are sensitive to the fire retardents. Also note that XPS is blown with HFC134a (sometimes straight, sometimes mixed with other HFCs), which has a very high global warming potential. By contrast EPS is blown with pentane, with only 1/200 the global warming hit. After 50 years the R value of XPS will have been reduced to that of EPS anyway, whereas the EPS R-value is stable. So, if a polystyrene is to be used, it's better to go with EPS. Better yet, use polyiso, which is also blown with pentane, and has a higher R value than XPS. (If you can find a roofing foam recycler 2" roofing iso can be cheaper than batts.)

2) Running the slab vapor retarder up the wall is the right way to go- the french drain is for sub-slab drainage, not flooding from above. (If you have a high flood risk you shouldn't be using wooden studs in the first place.)

3) R10 foam would meet IRC code min for Albany, but R10 foam + studwall w/R11-R15 batts would still be cost effective in the long run. If doing the turned-stud approach it's worh spliting R13s and compressing them between the 1.5" thick studs.

At R5/1" xps sub-slab is way better than nothing, and is sufficient to protect the rug & subfloor from condensation moisture, but R10 would be cost effective long term, if you have the headroom for it (but you probably don't). See Table 2, page 10. For reference, Albany is on the boundary of US zones 5 & 6:

You can use roofing iso (which is higher density, with better compressive strength) under the subfloor as long as you have poly between it and the slab, but not the foil faced goods.

4) Mold is a huge concern if you're putting wood (even pressure treated) against concrete. This includes the bottom plate of your sub-wall. Run the floor foam all the way to the foundation wall, and put the stud plate on top. (Again, it's not structural, and a 2x2 bottom plate would be fine if going with the turned-stud approach.)

5) Great Stuff is fine in the quantities you're talking. Most of the outgassing is during application, but if you're really concerned, fiber reinforced duct mastic applied 3/16" thick 2" either side of the seam is a pretty good seal with no VOCs or HFC outgassing.

6) The amount of moisture you'd have coming up from the uninsulated slab in the utilitiy room is miniscule, and can be reduced considerably with masonry sealers. You need not set the dehumidistat below 50% RH- it takes a sustained 65%+ @ 70F to have a serious mold issue, but holding the line 50% eliminates dust mite reproduction too.

You may want to read this bit o' bloggery too, as a second opinion. (You'll note that he mis-quotes the IRC on basement wall values- it's R10 if all foam for zones 5+.)

Assuming your basement is dry - I think you are getting a little carried away.

"I thought you had to use PT for sill applications (I think my town requires it actually). " - I said use PT for the bottom plate (that is the sill plate). You can use regular fir 2x4's for the wall studs. You need a bottom plate. The fir 2x4's are just fine against the block wall.

I believe the building codes (ask your inspector) require R-10. I believe that 3" of XPS is R-10. The studs should be attached to the top/bottom plate - you don't need to attach the studs to the wall. Forget that 'thermal break' stuff - it's wood. A stud has an R value of about 4.5 - which isn't bad - almost 1/2 of R10.

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XPS is R5/inch when new, R4.2/inch when fully outgassed (in 50 years or so.)

You CAN'T for get the thermal break, especially if you go with 2" foam between the studs. Most species used for framing in the northeast run ~R1.2/inch at 15% moisture content but will be far lower if wicking moisture from the wall. That would make a 2x4 about R4.2 if dry, but in terms of the thermal bridging it's a 3-D model:

Only the amount of wood penetrating the insulation counts- the rest of the stud to the interior of the foam is at room temp, and can be considered a heat sink. That's why 2" of foam between studs is a thermal-performance DISASTER, since you only have 2" of wood (R2.4, in your driest of dreams) not R4+.

But with a full cavity fill it's pretty much a 2-D model, and wood fraction can be consider at it's full (if still pretty low) value.

Code is R10-continuous insulation, R13-cavity if broken by thermally bridging studs. But doing both gives you about an R20 whole wall performance for pretty cheap. If you want to really cheap out, go with 1" continuous foam + R13 full-depth batt insulated wall, which performs at about R14-15 whole-wall. In climate Zone 5 even 1" EPS is sufficient for dew point control on an R13 batt wall.

I have never worked in a basement where the concrete walls were flat or plumb, so the stud walls are always built to the inside of the existing concrete walls without touching them. 2" XPS is screwed tight to the masonry using tap-cons and furring strips.

I like Dana's idea of building on top of the flooring insulation. I have been cutting Durock cement board into 3-1/2" strips and using Red Head anchors to bolt the bottom plate through the strip and into the floor. Our basements don't have enough headroom to insulate and finish floors. Admittedly, tile over concrete is pretty cold most of the year.

My 1920s vintage poured basement was surprisingly flat & plumb, which allowed me to use 3" reclaimed roofing iso and 1x furring TapConned to the concrete 24" o.c. and hang the gypsum on the furring, no studwall at all.

It's shocking how wavy & out of plumb my inlaws' circa 1980 raise-ranch house is by comparison (and not just the foundation)- on the first walk-through it was almost as if I'd been imbibing(!).

The fewer the fasteners penetrating the foam the better. If installing a much-straighter studwall use shims to provide the pressure for trapping the foam rather than through-screwing it. If insulating the cavities in a 2x4 wall with an uneven backing that makes it more than 3.5" deep, use unfaced R19s and COMPRESS them for a full-depth fill, to limit the amount of performance robbing convection that would otherwise be happening. An R19 compressed to 3.5" is still R13- in fact an R19 is essentially a "fluffed" R13- they're exactly the same weight per square foot. They may well be the crummiest product in the fiberglass insulation biz, but it's an appropriate solution to this problem.

First, thank you everyone for your replies. Rest assured as I become more well educated I will def. hang around and pay it forward. Hopefully I can save you all from answering the same questions twice, or three times! LOL!

First if you put the studs against the concrete you'll have a HUGE thermal bridge undercutting the performance of the foam, and provide a wicking path for moisture.

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Yeah, I did some more research based on the suggestions on here and it looks like using the xps all the way around is the way to go. I plan on using adhesive and taping the seams, but am unsure of how I want to frame right now, whether using 1X3 and attach through foam into concrete or full stud wall. My concerns is that screw heads, as small as they are, are opportunities for moisture to condense. If I float a studwall I don't have to worry about fasteners transmitting cold from the concrete?

You can then insulate the stud bays with fiberglass or rock wool, as long as it is unfaced goods.

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Would I was a little afraid to use ANY fiberglass down there but I found a few videos online where people in the extreme North (New Hampshire, Canada) use fiberglass in the studwall and then basically seal the studwall in Mil plastic? Could I use this instead?

If the finish floor is carpeting, you need to use at least 23/32" t & g subflooring, not half-inch plywood. You can get away with thinner stuff if the finish flooring itself has some structural strength

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Unfortunately, I only have 78" from concrete to the bottom of a 20' soffit that houses a main beam and HVAC ducting so I only have 2" to work with. If I go with 3/4" T&G then I can only use 3/4" foam. This cuts my R even further. I would prefer NOT to go with a laminate floor, but seems my options are that with a 1/2" subfloor and R5 foam or Carpet with a 3/4" T&G subloor and R3.5? foam?

Cut'n'cobbled foam in this area is best sealed with a closed cell foam such as Great Stuff or if you have a lot of it, FrothPak (the 12 board foot kits are pretty cheap compared to the amount of Great Stuff you'd be going through, and the blowing agent used isn't quite as hazardous to the installer.)

2) Running the slab vapor retarder up the wall is the right way to go- the french drain is for sub-slab drainage, not flooding from above

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So are you indicating that it WOULD be ok to run 6 Mil UNDER the 3/4" or 1" XPS on the floor? Most of what I read indicated that ANY Mil plastic in this project would pretty much be the way to a disaster. and run it and the xps all the way to the wall OVER the drain and bring the wall panels down on top of that? Then foam the joint and stud after?

You can use roofing iso (which is higher density, with better compressive strength) under the subfloor as long as you have poly between it and the slab, but not the foil faced goods.

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All I have been able to find is XPS and Foil Polyiso. Any suggestions?

Finally, I have heard that having a vapor barrier on the exterior of the foundation wall (which I would hope this has) and a vapor retarder on the inside (2" XPS) can force moisture to follow the inside of the foundation wall up directly to the sill plate. I have a pretty good idea of how to do the floor and up to the top of the foundation walls, but have no practical knowledge for how to handle above the foundation walls near the sill of the house? Do I put sections of XPS in here as well and foam around it?

Actually, the entire reason XPS foam is recommended is because the foundation walls below grade MUST dry towards the inside of the basement. The XPS allows the walls to this and at the same time provides insulation. The mistake some builders have been making for years is using a vapor barrier like poly sheeting on the walls, which traps the moisture and is a sure way to cause mildew/moisture/rot.

A basement can be functional living space, even here where it is below freezing for months at a time. It's just a matter of understanding how to make it work.

Look - these are basements. The homes were not built (ignorance / cost) with insulation in mind. There are a myrid of design / implementation probelms:

I just don't understand why some guy's want to go 'crazy' with insulating a basement, when the home was not built for it, in the first place.

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DougB, the answer to this is because I am buying a 945 square foot townhouse for $160K which is slightly below the average price for a property like this. This townhome has a basement, which most of these in this area don't. I NEEDED that because we have a roommate whose rent payment will pay 2/3rds of the mortgage and will be living there until he finishes school (3-5 years) I NEED to finish this as a living space so we can have separate spaces and if I DOUBLE the square footage of the property I should be able to DO the work for $15K and have been told by multiple people that that square footage in that area will be worth upwards of $190K, right now, not withstanding appreciation. GREAT INVESTMENT for my situation. I NEED to make this livable space and I want to do it right. I know MANY people have finished basements into bedrooms and living spaces successfully, but I also have seen many in my work who have not.

If you don't want moisture on the wall - then excavate, and coat the wall with an asphalt sealer, followed by poly and more sealer. Same with the air gap at the sill plate, and all the other problems.

What an active imagination about moisture / mold , etc, in a dry basement. This is why not to make it air tight - so it can breate.

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I think that the foundation may already BE coated. As I said, it looks like the basement has always been BONE DRY! Still, as you point out, it IS a basement so there IS more moisture than in a normal living space. If its being used as a basement, this is not a problem. Because it is not, I am just trying to make sure I am managing that additional moisture responsibly for myself and whoever may purchase the property from me in the future. I mean, they may put their kids down there.

As a degreed engineer - and I don't mean to say that because I have a degree, that my opinon is worth any more than anyone elses... but....

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Everyone's opinion is welcome and appreciated and I especially appreciate the opinions of those who are educated to know because, frankly, you could be making money answering my questions instead of doing it for free on a forum! LOL! I appreciate everyone's willingness to help!

The funny part is, MOST of the informed opinions I have heard have been very close to each other. Using XPS, and a lot of the framing details are similar. I had the guys at the town tell me I was "going overboard" too with what I am considering and the $ I am willing to spend. I appreciate their advice as well, but they are there to make sure I meet required minimums. If I'm willing to overdo it, and it's not going to CAUSE any problems, rest assured, it's because I am willing to spend more $$ for peace of mind.

Actually, the entire reason XPS foam is recommended is because the foundation walls below grade MUST dry towards the inside of the basement. The XPS allows the walls to this and at the same time provides insulation. The mistake some builders have been making for years is using a vapor barrier like poly sheeting on the walls, which traps the moisture and is a sure way to cause mildew/moisture/rot.

A basement can be functional living space, even here where it is below freezing for months at a time. It's just a matter of understanding how to make it work.

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Cacher_chick, The poly on the wall is out. I totally agree. I guess two of the questions I am trying to answer is:

A) Will the XPS allow enough ventilation to not direct any moisture that MAY find its way into the wall up to the sill.

B) I had a link in my previous reply for John Mansville Comfortherm insulation. Its basically rolls of insulation encapsulated in poly. Now, I HAVE seen several recommendations that say to use fiberglass inside of your studwall, but I don't feel entirely comfortable with that. I know poly is a no-no, but as long as the poly around the fiberglass remains intact, it should be fine, right?

I have no idea what the permeability of that insulation is. If it is wrapped in poly and tight fit, it seems like the wrong thing to do IMO. Even the wrong paint on the wall can act as a vapor barrier, so some research needs be done before you add anything different to your stack of materials.

Cacher_chick, The poly on the wall is out. I totally agree. I guess two of the questions I am trying to answer is:

A) Will the XPS allow enough ventilation to not direct any moisture that MAY find its way into the wall up to the sill.

B) I had a link in my previous reply for John Mansville Comfortherm insulation. Its basically rolls of insulation encapsulated in poly. Now, I HAVE seen several recommendations that say to use fiberglass inside of your studwall, but I don't feel entirely comfortable with that. I know poly is a no-no, but as long as the poly around the fiberglass remains intact, it should be fine, right?

Thanks again to everyone for the support.

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A> If you have 18" of exposed exterior on foundation you have plenty of capacity for ground moisture to dry toward the exterior.

B> Any insulation on the interior side of the foam needs to be allowed to dry toward the interior. Air-permeable batts wrapped in plastic WILL eventually take on some moisture, but would take forever to re-dry.

DougB: As a degreed engineer why not do the math? Just the above-grade portion of the foundation in most homes in MN would represent on the order of 20,000 BTU/hr of heat load at the outside design condition (in Albany NY it would be more like 15K) and a large fraction of an otherwise reasonably insulated home's heating fuel use. In a 965' town house in Albany wedged between 2 other units it's probably half that. Say you have only 60' exterior basement perimeter, with 2' of exposed above grade concrete with U-factor of 1.0, and another foot of band joist with a U-factor of 0.5, at Albany's 99% outside design temp of -2F and an inteior temp of 68F that's a 70F delta-T on 120' of U1 concrete for 8400 BTU/hr plus 60' of U0.5 wood for another 2100 BTU/hr, or 10,500 BTU/hr total. If you insulate that to R20 you're in the weeds at well under 1000 BTU/hr- the place stays warmer & drier, both winter and summer.

Finishing it without insulation would be a mold hazard (and in some places, a code violation). Also, water sealing the wall from the exterior of does not keep the concrete from wicking up from the footing, which is best allowed to dry toward the interior, especially if you have less than a foot of exposed exterior to dry toward. If you use foil-faced iso, leave the bottom 8-12" either uninsulated, or insulated with unfaced EPS.